Nowadays, the high speed access network is mainly based on a technology of xDSL (Digital Subscriber Line). However, the limited bandwidth-distance product of a copper-based network will be a bottleneck for future multimedia-services. Foreseeing a rapid growth of the multimedia traffic and a trend of service convergence, it is obvious that a fiber-based access network will be an ultimate solution.
Time-division-multiplexed passive optical network (TDM-PON) systems such as broadband PON (BPON) and Ethernet PON (EPON) are being widely deployed in current generation fiber-based access networks for providing broadband access, offering triple play services including video, data and voice. In the near future, it is generally agreed that wavelength-division-multiplexed (WDM)-based access networks will be an enabler of the next-generation optical broadband access that requires large dedicated bandwidth and upgrade flexibility.
As the bandwidth-hungry video streams such as high-definition (HD) television become more and more pervasive in the access network, broadcast service is a very important issue in PON systems. Compared to the broadcast service, the multicast service can offer exclusive service to premium subscribers, thus generating more revenue. Broadcast can be easily realized in TDM-PON by power-splitting at a remote node (RN). However, such is more challenging for WDM-PON, due to the dedicated connection between the optical line terminal (OLT) and the optical network unit (ONU). To realize more flexible network functions, several schemes have been proposed to simultaneously deliver both point-to-point (p-t-p) data and broadcast/multicast data to subscribers in a WDM-PON.
There are two key issues facing transmission of both p-t-p and multicast data simultaneously in WDM-PON: 1). how to multiplex the p-t-p and multicast traffic; and 2). how to enable/disable multicast traffic flexibly. Subcarrier multiplexing could be employed to superimpose the multicast data on the p-t-p data (less than 1.5 Gb/s was demonstrated). However, a broadband modulator and oscillator, with a bandwidth of several times larger than the signal bit rate, are needed at the OLT and/or ONU sides. Recently, two schemes based on orthogonal modulation were proposed to superimpose a 10-Gb/s multicast data stream on the conventional 10-Gb/s downstream p-t-p data. As the upstream transmission was realized by re-modulating a part of downstream NRZ using a high extinction ratio (ER) when the multicast traffic was disabled, the bit rate of the upstream signals was limited to 2.5-Gb/s. To solve this problem, a delay based multicast control scheme has been proposed. However, the multicast traffic cannot be effectively disabled when its bit rate is lower than that of the downstream data.